HR: 0830h
AN: OS51C-0879    [PDF]
TI: Seismic Study of Natural Gas Hydrates at Northern Hydrate Ridge, Cascadia Accretionary Complex
AU: * Petersen, J
EM: jpetersen@geomar.de
AF: GEOMAR, Wischhofstr. 1-3, Kiel, D-24148 Germany
AU: Papenberg, C
AF: GEOMAR, Wischhofstr. 1-3, Kiel, D-24148 Germany
AU: Klaeschen, D
AF: GEOMAR, Wischhofstr. 1-3, Kiel, D-24148 Germany
AB: The abundance of gas hydrates at Hydrate Ridge, located landward of the deformation front of the Cascadia accretionary complex, makes it an ideal research target for studying the dynamics of hydrate formation and dissociation processes. Thus, a detailed seismic investigation of Hydrate Ridge was carried out within the HYDGAS project aboard the R/V Sonne in September 2000. The major aim was the quantification of the amount of solid gas hydrate and free methane using multi-frequency sources and streamer (deep tow and surface SCS) and ocean bottom (OBH/OBS) receivers. Core and log data collected during ODP legs 146 (Westbrook et al., 1994) and 204 (Trehu, Bohrmann et al., pers. comm.), MCS data from survey OR89 (MacKay et al., 1995; Trehu et al., 1999) and side-scan sonar data (SO165-1, 2002) were available for an integrated data analysis. Here, we present results from the northern ridge, the summit of which reaches $\sim$590 m water depth. A total of 34 densely spaced GI-Gun profiles (line spacing 400 - 1500m) were acquired with a single channel surface streamer. Analysis of the BSR depth distribution shows two major regions of an anomalous shallow BSR, interpreted as a result of upward fluid migration and associated high heat flow. These areas are located westward of ODP drill site 892 and near the topographic summit. Site 892 penetrated a eastward-dipping thrust fault at a depth of $\sim$100 mbsf along which deep-seated fluids rise feeding a carbonate chemoherm structure located 350m further west (Linke et al., 1994, Davis et al., 1995). The mapped reflection amplitudes of the BSR show disrupted or disturbed BSR signatures interpreted as zones of upward migrating fluids. Comparison of the BSR depth and amplitude anomalies with side-scan sonar data shows good correlation of chemoherm structures and active venting fields. An AVO analysis of the wide angle OBH/OBS data using P and PS converted waves was done to determine the elastic properties of the sediments beneath the instrument locations. A strong PS conversion at the seafloor might be enabled by an increase in the shear modulus due to outcropping gas hydrate or carbonate crusts. The concentrations of gas hydrate and free gas derived from the OBS data are used to calibrate the P wave amplitude data and allow an estimate of the regional hydrate/gas distribution.
DE: 0910 Data processing
DE: 0935 Seismic methods (3025)
DE: 3025 Marine seismics (0935)
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting